Molded plastic articles are becoming widely accepted as a replacement for metallic and glass articles. One advantage associated with molded plastic articles is the integration of several components into one article, thereby reducing the number of assembly operations. In other words, an article that previously was comprised of several components bonded or joined together may be manufactured in a one step, molding operation. One inherent problem that has resulted from the advent of this practice is the ability to print upon the resulting complex (concave, convex, etc.) surface shape of the article. Printing is desirable since other means for disposing images are timely and the use of several 2-dimensional printing concepts, namely screen-printing and pad-printing, have been extended to meet this need with only limited success.
Screen-printing is a known commercial process and is described in greater detail below. Screen printing is limited in the complexity of the surface upon which may be printed. This technique represents a very economical method for printing onto a “flat” substrate. Screen-printing has been applied to curved surfaces through the implementation of a technique known as in-mold decoration (IMD). In this technique the printed image is applied via screen-printing to a “flat” film. This film is then held via vacuum to the surface of the mold. The film becomes part of the surface of the article upon the injection of the plastic material into the mold. Major difficulties associated with the use of this technique are the registration of the decoration on the article's surface and a limitation in surface complexity of the article. Decoration registration requires accurate positioning of the film into the mold for each article reproduction. Surface complexity is limited by the ability of the film to conform (e.g., stretch) to the shape of mold to be incorporated as part of the article's surface.
Pad-printing is also a known commercial printing process and is described in greater detail below. Pad-printing is a printing process which uses a tampon and a cliché to stamp or print onto a convex curved surface. In fact, pad-printing or tampography is a form of indirect or offset gravure printing that is accepted by the automotive industry for the decoration of interior components. Pad or tampon printing is an economical technique capable of providing fine line (32 micrometer) resolution on both curved and uneven surfaces. However, this technique is limited in the degree of complex curvature, radius, and size of the substrate to be printed, as well as in the design of the substrate's edge up to which one may desire to print.
Significant differences between screen-printing and pad-printing exist with respect to the composition of the ink utilized. Typically, the inks used in these two application methods are very different in their solvent make-up. In order not to dry in the screen, the ink formulations used in screen-printing contain solvents whose evaporation rates are lower than those used in pad-printing inks. In pad-printing ink formulations, solvent evaporation is utilized to modify rheological properties and surface tension in order to provide a “tacky” film on the pad during transfer. Thus many commercial screen-printing and pad-printing inks will not optimally function in a printing process that combines both conventional printing techniques into one method, such as MIT printing.
Therefore, there is a need in the industry to formulate inks that not only may be used in conventional screen-printing or pad-printing application techniques, but also will function in processes that incorporate both conventional techniques into one process.